The invention relates to call forwarding functionality.
In telecommunications networks call forwarding functionality forwards a call to a predetermined number in case of a busy line, no reply, or unconditional call forwarding, for example. In order to avoid infinite loops in call forwarding and consequent overloading of the network, the number of successive times a call can be forwarded is usually limited. In some countries or networks, call forwarding is allowed only once per call. In this case, information is transferred as an attachment to the signaling, which indicates whether the call has been forwarded or not. When a forwarded call is indicated, re-forwarding of the call is prohibited. Hence, two subscribers B and C who have designated call forwarding to each other would not cause a call made by subscriber A to subscriber B to be forwarded back and forth between the numbers of B and C. In another solution, call forwarding restriction is implemented with the aid of a call forwarding counter CFC, which is transferred between switching centers. CFC indicates the number of times call forwarding has already been executed for the call. A maximum number set for the CFC restricts the number of times a call can be forwarded, e.g. five successive times for call forwarding per one call. Usually the maximum number is some value from 1 to 5.
FIG. 1 of the attached drawing shows a simplified block diagram of a GSM mobile communications system (Global System for Mobile communications). The mobile station MS is connected via a radio path to a base transceiver station BTS, in FIG. 1 to the base station BTS1. A base station sub-system BSS consists of a base station controller BSC and the base stations BTS controlled by it. A mobile services switching center MSC usually controls several base station controllers BSC and is connected to other mobile services switching centers and a GMSC (Gateway Mobile services Switching Center). Via the GMSC, the GSM network is connected to other networks, such as the PSTN (Public Service Telephone Network), another mobile communication network PLMN, or the ISDN network. Subscriber information is stored permanently in the home location register HLR and temporarily in the visited location register VLR. The operation of the entire GSM system is monitored by the operation and maintenance center OMC.
FIG. 2 of the attached drawing shows call routing in the GSM network as a signaling diagram. In the following, call forwarding is described with reference to this figure. In the example of FIG. 2, a call is transferred from the network to the gateway mobile services switching center GMSC with the setup-message IAM 20 (Initial Address Message). As an attachment to the IAM-message, the telephone number of the called subscriber and the information about call forwarding or the call forwarding counter CFC are transferred. The GMSC sends a request to the home location register HLR of the called subscriber for routing information in the SRI-message 21 (Send Routing Information). Information about call forwarding set for this called subscriber number is contained in the subscriber information in HLR and/or VLR. Unconditional call forwarding can be performed by the HLR, whereas call forwarding on Busy can only be carried out by the VLR in charge of the subscriber. Conditional call forwarding, such as call forwarding on Not Reachable (CFNRc) and call forwarding on No Reply (CFNRy), can be performed either by the HLR or the VLR.
When the HLR initiates call forwarding, the HLR sends the forwarded-to number to the GMSC in SRI ACK-message 24 (Send Routing Information Acknowledge). After receiving this number to be used in call forwarding, the GMSC examines the information about call forwarding received with the IAM-message. If the information indicates that the call has already been forwarded and the call can be forwarded only once, the call is not forwarded second time. When a call forwarding counter CFC is transferred with the IAM-message, the GMSC compares the value of the CFC to the maximum value set for call forwarding. For example, if the value of CFC is 3 and the maximum value is set at 5, the CFC value is increased by one and the call is forwarded to the number given for forwarding. If the CFC value equals the maximum set value, the call is not forwarded once more. If call forwarding is still allowed, the call is forwarded to the new number, e.g. to another switching center VMSC in IAM-message 25 which contains the information about call forwarding or the new CFC. If call forwarding is restricted, the call is normally released/disconnected. In call forwarding initiated by the HLR, messages 22 and 23 in FIG. 2 are not needed.
When VLR initiates call forwarding, the HLR sends the PRN-message 22 (Provide Routing Number) to the subscriber""s current visited location register VLR. The VLR provides the HLR with mobile station routing number MSRN (stage 23), which HLR forwards to GMSC in SRI ACK-message 24 (Send Routing Information Acknowledge). Based on the routing information, the call is routed to the visited mobile services switching center VMSC, where the VLR provides the number to be used in forwarding the call. The switching center examines if call forwarding is still allowed for this call using the information about call forwarding or the call forwarding counter CFC as described above. If call forwarding is still allowed, the call is forwarded to the given number along with the information about call forwarding or the new CFC as described above.
The examination of call forwarding information can just as well be performed by the HLR when the information about call forwarding or the CFC is transferred to the HLR in connection with the routing number request. In that case, the examination is carried out in a manner similar to that described above, usually both in the switching center and in the HLR. As a result call forwarding is either allowed or denied.
FIG. 3 shows a block diagram of the units of the switching center which are essential for call forwarding functionality. FIG. 3 illustrates the use of call forwarding counter CFC for call forwarding restriction. The CF (Call Forwarding) handling unit 31 receives the parameters relating to the forwarding of the call in question and compares the CFC value received to the maximum value retrieved from max_CF storage 33. Call forwarding information of the called subscriber number is retrieved from the subscriber information of HLR via HLR request unit 35 and/or from the VLR 37, which can be located in proximity to the switching center. After comparison of the CFC value, the call is either forwarded, when CFC is below the maximum value set, or disconnected, when CFC is equal to the maximum value. When forwarding the call, the CF handling unit 31 increases the CFC value by one.
The problem with the call forwarding restriction described above is that access through call forwarding to numbers with ensuring an answer rather than possible further forwarding, such as voice mail, is not possible. Therefore, a call made by subscriber A to subscriber B can be forwarded to the mobile phone of subscriber B but cannot then be forwarded further on to the voice mail, if only one call forwarding is allowed.
The object of this invention is to facilitate call forwarding when call forwarding restriction takes place.
This is achieved by a method and an arrangement according to the invention characterized by what is stated in the independent claims. Special embodiments of the invention are presented in the dependent claims.
The invention is based on the idea that call forwarding is allowed despite call forwarding restriction when the number to be used in forwarding the call meets special conditions set beforehand. The number is thus compared with the preset conditions and depending on the result of this comparison the call is either forwarded or not.
The advantage of call forwarding according to the invention is that call forwarding can be allowed to certain telephone numbers despite any call forwarding restrictions.
Another advantage of the invention is that call forwarding is flexible and conditions for additional call forwarding can be custom-tailored.